Heater for an indirectly heated cathode



Feb. 14, 1967 SHAW 3,304,459

HEATER FOR AN INDIRECTLY HEATED CATHODE Original Filed May 21, 1964 I/VVEN 70/? BEVERLEY A. SHAW United States atent (lfilice 3,304,459 HEATER FOR AN INDHRECTLY HEATED CATHUDE Beverley A. Shaw, North Reading, Mass., assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Continuation of appiication Ser. No. 369,184, May 21,

1964. This application June 13, 1966, Ser. No. 557,324

2 Claims. (Cl. 313337) This is a continuation of Serial No. 369,184, filed May 21, 1964-, now abandoned.

This invention relates to thermionic cathodes for electron discharge devices and has particular reference to improved heaters for indirectly heated cathodes.

Heater structures for cathodes of electron discharge devices such as traveling wave tubes and power tubes, for example, conventionally employ fine filamentary wires which are subjected to tortuous bending or coiling or the like in order to obtain a combination of resistance and current which is practical for present power supplies and for the energy requirements of the cathode. This is undesirable because of the fact that at temperatures at which the heater must operate, all such heaters will have finite life due to the ease with which the tungsten, or other material from which the heater is fabricated, can be crystallized across the diameter of the fine filamentary wire.

These disadvantages are overcome in the present invention by the provision of an improved heater structure which has a novel geometrical aspect and which permits operation at lower heater temperatures, both of which factors result in orders of magnitude of improvement of heater efficiency and heater life.

The advantages of this invention are achieved by a heater such as a noninductively shaped helix of tungsten or tungsten alloy which may be formed as by photoetchin or the like so as to possess characteristics for use as a high temperature heater. The resultant heater coil is sandwiched between two sapphire plates and mounted as a unit adjacent the rear surface of an electron emitting element to be heated. The sapphire plates provide mechanical support for the heater and thus prevent damage to the heater wires such as often occurs when the device is subjected to mechanical shock or vibration, especially when the tungsten has been recrystallized by heat.

Heat or infrared energy from hot tungsten has been found to be about 90% in the l6,u, range at 1100 C., and the infrared transmission of sapphire in the 1-6 range is extremely high. Thus, most of the heat from a tungsten heater will be transmitted through the sapphire plates. The sapphire surface most remote from the emitter is preferably coated with a highly infrared reflecting coating so that most of the heat from the heater will be directed toward the emitter,

Such heater devices are readily adaptable to modification and may be made, for example, with a second coil located in the spaces of the first coil to provide a reserve, supplemental, or alternate heater, or the device may be provided with two parallel coils spaced by a layer of sapphire. In such plural heater devices it is possible to extend cathode life by varying cathode temperature and tube operating levels simply by appropriately connecting the different heater leads.

In accordance with this invention it has been found that a structure as described provides a far more efficient cathode furnace than known prior art devices so that the operating temperature of the heater for a given cathode temperature is very close to that cathode temperature. This results in greater overall efficiency and savings.

Other advantages of this invention will become apparent from the following description taken in co'nnec tion with the accompanying drawings, wherein:

FIG. 1 is a fragmentary enlarged axial sectional view of a cathode structure embodying the invention;

FIG. 2 is an elevational view of one form of a heater element in accordance with the invention;

FIG. 3 is a top elevational view of a complete heater structure;

FIG. 4 is a transverse sectional view of the heater structure taken on line 4-4 of FIG. 3; and

FIG. 5 is a transverse sectional view similar to FIG. 4 illustrating a modification of the invention.

Referring more particularly to the drawings wherein like characters of reference designate like parts throughout the several views, the cathode structure of FIG. 1 comprise a planar type device having a cup-shaped support 10 on the open end of which is fired a focusing electrode 11. The support 10 is mounted by suitable means such as by being affixed to the end of a hollow dielectric cylinder 12 or the like which actually may be part of the tube envelope.

Within the support 10 is located a cathode 13 which preferably is formed as a cu=p-shaped element 13 having an annular side wall 14 and a concave base 15 which is the electron emitter. The emitter 15 is formed of material having the ability when heated to give off copious supplies of electrons, such as tungsten impregnated with barium aluminate, for example. The cathode 13 is mounted within support 10 adjacent the inner surface of the focusing cup 11 as by a number of supporting rods or pins 16.

Within the confines of the annular side wall 14 is located a heater structure 17 which is a disclike device sealed throughout its periphery to the wall 14. The heater structure 17, to be described more fully in connection with FIGS. 2-5, includes a heater element or wire 18 the opposite ends of which are afiixed to two leads 19 and 20 which extend externally of the tube for connection to a suitable source of filament voltage. Lead 20 conveniently is a tubular member which has one end connected to support 10, with its other end being adapted for connection to ground. The grounded end of the heater element 18 is conveniently connected directly to this lead. The second lead 19 is an elongate wire which extends longitudinally within the tubular lead 20, being insulatedly supported therein as by insulating members 21. Lead 21 is connected directly to the ungroundcd end of the heater element.

Referring more particularly to FIG. 2, the heater element 18 is a spiral of tungsten, platinum, molybdenum or other suitable material which may be photoetched from a sheet or strip of the selected material. The element may be of suitable size depending upon the tube in which it is to be used. For example, a one-inch diameter heater may be about .005" thick, a /8 inch diameter heater may be about .001 thick, and a tiny A; inch diameter element may be about .0005" thick. The element is preferably photoetched since other methods of fabrication sometimes cause cracks, deformations or other flaws in the material.

While photoetching may be done by any known process, the following procedure was successfully used in producing heaters from a sheet of tungsten six mils in thickness. The member is first coated with a photoresist such as the Kodak Metal Etch Resist (KMER) supplied by Eastman Kodak Company, and thoroughly dried. The member with the photoresist thereon is there-after exposed on its opposite surfaces to light having peak intensity at about 36 60 A. through aligned mirror image negatives for about two minutes. The exposed image is then developed by soaking in a kerosene-like petroleum 3,304,451 aatentea rea. 14, 1967- distillate for about two minutes, rinsed first with xylene and then with water, and then quick dried.

At this point the areas outside the heater pattern are masked and the member then etched in an electrolyte comprising about 500 cc. water, 20 grams sodium hydroxide, and 400 cc. 30% hydrogen peroxide which was cooled to about 20 C. Etching is done for about seven minutes at a current which was raised from 2 amps to 8-10 amps whereupon the heater becomes separated from the major portion of the member from which it is formed. The resultant heater is then rinsed in water and dried, after which the resist coating is removed by soaking in a suitable solvent for about five minutes, scrubbing gently, and rinsing in water. The finished heater then is ready for assembly into the heater structure shown in FIGS. 3 and 4.

Although the heater may be formed as a single spiral, to provide a noninductive heater the configuration takes the form of a double spiral or a single spiral doubled back upon itself as shown in FIG. 2. In the latter case the outer ends of the two spirals are joined and the inner adjacent end portions are bent down to form two separate spaced tabs 22 and 23. The heater 18 is sandwiched between two sapphire plates 24 and 25 as shown in FIG. 4, with the two tabs 22-23 protruding through openings provided therefor in one of the plates. The tabs 22 and 23 are connected to the leads 19 and 20 respectively in the cathode illustrated in FIG. 1.

When voltage is applied to the heater 18 through leads 1920 and tabs 22-23, the heater will radiate infrared energy in all directions. About 90% of this energy is in the range of about 1-6p. when tungsten is used as the heater and is operated at about 1100 C., for example. Since sapphire transmits an extremely high percentage of infrared energy in the l6 range, the emitter will become heated to the point of electron emission. The heater structure 17 is, therefore, located relatively close to the adjacent surface of the emitter and may, in fact, conform to the shape of this surface.

Since the heater is required to operate in a unidirectional mode, the outer surface of the sapphire plate 24 most remote from the emitter is coated with a highly infrared reflecting coating 31 whereby infrared energy impinging upon it will be reflected back through the structure to the emitter 15. Coating 31 may be tungsten, molybdenum or zirconium deposited by means such as vacuum evaporation or thermal decomposition.

It will be apparent that a structure of this type will provide eflicient mechanical support for the delicate heater element 18. The device withstands relatively severe vibrations and mechanical shocks which damage conventional heaters. Such support by the sapphire plates is, furthermore, accomplished without materially affecting the radiant infrared energy from the heater and permits reflection of backward directed energy toward the emitter without requiring the use of additional separate reflecting elements.

It has also been found that the structure is a far more eflicient furnace than known structures, and this permits the operating temperature of the heater to be maintained very close to that of the cathode, achieving economy in heater wattage. This latter feature provides considerable increase in heater life since recrystallization of tungsten is a sensitive function of temperature.

This invention readily adapts itself to multiple heater devices such as shown in FIG. 5 wherein a first heater 26 is located, as described above, between two sapphire plates 27-28, and a second heater 29 is disposed between of the plates, plate 28 in FIG. 5, and a third sapphire plate 30, which is preferably coated on its exposed surface with a layer 32 of metal for heat reflection. Alternatively, a second heater may be located within the coils of the first heater. In such cases the second heater may be a reserve in case the first heater burns out, or it can serve for alternate connections when properly connected into filament control circuitry, and the two heaters may be connected in parallel or in series, as desired. Thus, during the life of a tube containing such a multiple heater structure, it is possible to vary and control heater life, cathode temperature, and tube operation levels as desired simply by proper connection of control circuitry to the heater leads.

While the invention has been shown and described in connection with substantially planar heater and cathode structures, it will be readily apparent that the invention may be applied to cup-shaped or cylindrical structures as well. For example, the heater sandwich may be comprised of machined cylinders of sapphire and photoetched tungsten tubes.

While sapphire has been shown and described as the material from which the infrared transmitting plates are formed, it is to be understood that other materials transparent to substantially all of the heat radiated by the heater may be used, such as high-fired alumina, or even regular alumina at high temperatures. Single crystal sapphire is greatly preferred since longer warmup times are required with alumina. Calcium fluoride or magnesium oxide may sometimes be used where the tube is operated at low temperatures.

From the foregoing it will be apparent that an improved heater structure has been provided in accordance with the described invention. It is to be understood, however, that certain modifications of the invention may be made by those skilled in the art without departing from the spirit of the invention as expressed in the accompanying claims. Therefore, all matter shown and described is to interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. 'In an electron discharge device, an indirectly heated thermionic cathode structure comprising a planar type electron emitter and a heater structure adjacent one surface of the electron emitter, said heater structure embodying a substantially flat spiral photoetched heater element having means for connection to a source of filament voltage whereby infrared radiation of about 16 microns in wavelength at about 1100 C. is emitted therefrom, said heater element being a delicate and fragile wire of a diameter not greater than about .005 inch, a pair of rigid support discs on opposite sides of the heater element, the heater element being sandwiched therebetween, said support discs being transparent to the infrared radiation emitted by the heater element, and an infrared reflective coating on the outer surface of the support disc most remote from the emitter.

2. A structure according to claim 1 wherein the heater element is tungsten and the rigid support discs are sapphire.

No references cited.

JOHN W. HUCKERT, Primary Examiner.

A. 1. JAMES, Assistant Examiner. 

1. IN AN ELECTRON DISCHARGE DEVICE, AN INDIRECTLY HEATED THERMIONIC CATHODE STRUCTURE COMPRISING A PLANAR TYPE ELECTRON EMITTER AND A HEATER STRUCTURE ADJACENT ONE SURFACE OF THE ELECTRON EMITTER, SAID HEATER STRUCTURE EMBODYING A SUBSTANTIALLY FLAT SPIRAL PHOTOETCHED HEATER ELEMENT HAVING MEANS FOR CONNECTION TO A SOURCE OF FILAMENT VOLTAGE WHEREBY INFRARED RADIATION OF ABOUT 1-6 MICRONS IN WAVELENGTH AT ABOUT 1100*C. IS EMITTED THEREFROM, SAID HEATER ELEMENT BEING A DELICATE AND FRAGILE WIRE OF A DIAMETER NOT GREATER THAN ABOUT .005 INCH, A PAIR OF RIGID SUPPORT DISCS ON OPPOSITE SIDES OF THE HEATER ELEMENT, THE HEATER ELEMENT BEING SANDWICHED THEREBETWEEN, SAID SUPPORT DISCS BEING TRANSPARENT TO THE INFRARED RADIATION EMITTED BY THE HEATER ELEMENT, AND AN INFRARED REFLECTIVE COATING ON THE OUTER SURFACE OF THE SUPPORT DISC MOST REMOTE FROM THE EMITTER. 